1. Field of the Invention
This invention relates to a method and apparatus which corrects for distortion in seismic records. More particularly the invention relates to a method and apparatus for removing phase distortion from deconvolved seismic data records generated with the use of vibrational energy.
2. Description of the Prior Art
In seismic prospecting, a seismic energy source is employed to generate a seismic signal transmitted into the earth. Part of this signal is reflected from subterranean interfaces back to the surface of the earth. There it is detected and converted to a seismic signal which is then recorded.
This recorded seismic field data is subjected to processing to improve its quality and thereby render it more meaningful to the geophysical interpreter. Filters used in such processing are designed so as not to change the time relation of reflective events. One type of this filtering, known as deconvolution, is in reality a form of inverse filtering which corrects for the previous filtering effects of the recording system and the earth itself. This inverse filter is determined from the record traces themselves. Reflective seismic field data such as that recorded in vibrated tests, after deconvolution in accordance with standard processing techniques, is still degraded by amplitude and phase errors not corrected by prior art methods.
A vehicle mounted vibrator includes a base plate lowered into contact with the earth and a heavy reaction mass coupled to said base plate. In a manner well known to the art, the vibrator is driven with an electronically generated frequency varying sine wave (hereinafter "pilot signal") so as to impart vibrational seismic energy into the earth. In U.S. Pat. No. 4,348,749 to Galbraith there is described a method for processing deconvolved seismic traces obtained from a vibrator to correct for phase distortion introduced in the deconvolution process. In this method a signal representing the minimum phase inverse of a synthetic version of the pilot signal is generated and convolved with such signal to obtain a first filtered function. The time reverse of this filtered function is then convolved with a signal representing the reflected seismic data after deconvolution in order to phase correct such traces. The use of Galbraith's pilot signal in the filtering process, however, continues to introduce phase errors. The reason is that Galbraith's technique ignores the difference between the pilot signal and the ground force actually imparted to the earth.
The prior art has recognized the distinction between the uncorrected signal pulse of a vibrator and the true ground force produced. As taught for example in U.S. Pat. No. 4,184,144 to Rickenbacker or in British Pat. No. 2,101,317 to De Kok, a correcting feedback signal is generated proportional to the combined mass acceleration of the base plate and reaction mass of the vibrator. The feedback signal may be used to adjust the actuator mechanism so that the phase and amplitude of the vibrator motion is representative of the pilot signal. Use of a pilot signal so corrected in processing of output field data, however, still fails to make full use of the information available by measuring the true ground force. It is therefore an object of this invention to provide an improved method and apparatus for correcting distortion in procesed seismic field data.
It is a more particular object of this invention to provide such a method and apparatus wherein such field data is generated by means of a vibrator. It is a still further object of this invention to provide a method and apparatus for correcting distortion in seismic data generated with a vibrator wherein the errors inherent in any standard processing technique are minimized.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow taken in conjunction with the drawings and appended claims.